Harken 45 mm Aluminum Loop BlockPart No. 3389 Black Magic Air blocks are a top favorite of Big Boat performance racers and cruisers. These lightweight, free running, powerful performers are used for sail controls that see lots of action: mainsheet, runner, halyard, and spinnaker systems. A center cage keeps Torlon roller bearings separated and parallel to reduce friction. The sheave center carries primary load, allowing a smaller, lighter weight block. The soft attachment Loop block